1. Field of the Invention
The present invention relates to a control apparatus for an internal combustion engine having first fuel injection means (an in-cylinder injector) for injecting fuel into a cylinder and second fuel injection means (an intake manifold injector) for injecting fuel into an intake manifold or an intake port, and particularly relates to a technique for starting the internal combustion engine using the fuel injected from the first fuel injection means.
2. Description of the Background Art
An internal combustion engine having a first fuel injection valve (in-cylinder injector) for injecting fuel into a combustion chamber of an engine and a second fuel injection valve (intake manifold injector) for injecting fuel into an intake manifold of the engine, and configured to stop fuel injection from the second fuel injection valve (intake manifold injector) when the engine load is lower than a preset load and to allow fuel injection from the second fuel injection valve (intake manifold injector) when the engine load is higher than the preset load, is known. In this internal combustion engine, the total injection quantity, i.e., a total quantity of the fuel injected from the fuel injection valves, is preset as a function of the engine load. The total injection quantity increases with the increase of the engine load.
The first fuel injection valve (in-cylinder injector) is arranged to open directly to the combustion chamber of the internal combustion engine, and injects the fuel pressurized by a fuel pump directly into the cylinder. The in-cylinder injector injecting the fuel directly into the cylinder of the internal combustion engine is used to improve the fuel efficiency and the like, by accurately controlling the mixed state of the air-fuel mixture within the cylinder by injecting the fuel during the latter stage of the compression stroke.
Since the in-cylinder injector is thus configured to directly inject the fuel into the cylinder, the following inconvenience may arise particularly during the cold start of the internal combustion engine. At the start of the internal combustion engine, it is often the case that the pressure of the fuel in a high-pressure fuel system supplying the fuel into the in-cylinder injector is lower than a prescribed pressure because of insufficient pressurization of the fuel by the high-pressure pump, since the high-pressure pump is driven by the driving force of the internal combustion engine. Thus, at the start of the internal combustion engine, if the fuel is supplied via the intake manifold injector and the in-cylinder injector at a fuel injection ratio therebetween that is set not taking the above-described point into consideration, the in-cylinder injector would inject fuel under the condition where the pressure in the high-pressure fuel system is extremely low immediately after cranking, in which case the fuel injected would be quite inadequate in atomization (or, too large in particulate size).
Such insufficient atomization of the fuel at the start of the internal combustion engine means that, when a prescribed quantity of fuel is supplied, mixing of the fuel with the air would not be conducted efficiently, which may lead to a decrease in concentration of the combustible air-fuel mixture in the vicinity of the spark plug, and thus, lead to failure in startup. If the quantity of the fuel supplied is increased for compensation, the lubricant oil may be diluted with the fuel, or black smoke (particulate matter) may be generated by combustion of the air-fuel mixture that is too rich locally as well as combustion of droplets, leading to deterioration of exhaust gas emission (particularly, HC and CO will increase).
Japanese Patent Laying-Open No. 2001-336439 discloses a fuel injection control apparatus for an in-cylinder fuel injection engine that enables appropriate setting of a ratio between the quantity of the fuel injected into a cylinder and the quantity of the fuel injected into an intake port taking account of the particulate state of the fuel injected into the cylinder at the start of the engine, to thereby improve the engine starting capability as well as the exhaust gas emission. The fuel injection control apparatus for an in-cylinder fuel injection engine disclosed in this publication includes an in-cylinder injector for injecting fuel into a cylinder, and an out-cylinder injector for injecting fuel into an intake pipe, and supplies the fuel at engine start using both injectors. The control apparatus includes fuel injection ratio setting means for setting a fuel injection ratio between the in-cylinder injector and the out-cylinder injector in a variable manner to obtain the total quantity of the fuel required at engine start, using the fuel pressure in the high-pressure fuel system supplying the fuel to the in-cylinder injector as a primary parameter. More preferably, the fuel injection ratio setting means uses the temperature condition upon fuel injection into the cylinder as another parameter, in addition to the fuel pressure in the high-pressure fuel system, and changes the fuel injection ratio such that the quantity of the fuel injected via the out-cylinder injector increases as the fuel pressure in the high-pressure fuel system is lower and the temperature is also lower.
According to this fuel injection control apparatus for an in-cylinder fuel injection engine, the quantity of the fuel injected from the out-cylinder injector is increased as the fuel pressure in the high-pressure fuel system is lower and the temperature is lower as well, to thereby restrict the quantity of the fuel supplied via the in-cylinder injector during the cold start of the engine. This can quickly increase the fuel pressure in the high-pressure fuel system, and accordingly, it is possible to realize atomization of the fuel injected via the in-cylinder injector in a short time.
During the cold start of the engine, it is common practice to decrease the quantity of the fuel injected via the in-cylinder and increase the quantity of the fuel injected via the intake manifold injector, as described in Japanese Patent Laying-Open No. 2001-336439. In doing so, however, the fuel injected via the intake manifold injector in the very cold state would adhere to the wall of the intake pipe or to the intake port of a low temperature, leading to a lean air-fuel ratio of the air-fuel mixture in the combustion chamber. The state where the air-fuel ratio of the air-fuel mixture in the combustion chamber does not become rich continues until adhesion of the fuel on the intake side is saturated, which results in a long time required for starting the engine. Meanwhile, simply causing the in-cylinder injector to inject the fuel as well would not solve the conventional problem as disclosed in Japanese Patent Laying-Open No. 2001-336439.
The condition to increase the quantity of the fuel injected via the out-cylinder injector in the state where the fuel pressure in the high-pressure fuel system is low and the temperature is low, as in the fuel injection control apparatus for an in-cylinder fuel injection engine disclosed in Japanese Patent Laying-Open No. 2001-336439, merely defines that the fuel is injected in a greater quantity from the intake manifold injector. That is, since the condition to increase the quantity of the fuel injected from the in-cylinder injector is not taken into consideration, it is still difficult to make the air-fuel mixture in the combustion chamber rich, resulting in a long starting time.